For power semiconductor devices, high withstanding voltage, as well as low on resistance and low switching loss are required, but silicon (Si) power devices, as a current main stream, are close to their theoretical performance Since silicon carbide (SiC) has a dielectric breakdown field strength larger, by about one digit than Si, the device resistance can be decreased theoretically by three digits or more by decreasing the thickness of a drift layer for maintaining withstanding voltage as low as to about 1/10 and increasing the impurity concentration by about 100 times. Further, since SiC has a band gap Larger by about three times than that of Si and high temperature operation is also possible, performance exceeding that of Si can be expected.
Among the power semiconductor devices using SiC, research and development have been progressed particularly for unipolar type Schottky barrier diodes (SBD).
Since the Si diode, as a current main stream, is a bipolar type that operates on two kinds of carriers of holes and electrons, a recovery current due to discharge of excessive minor carriers is generated upon switching to result in switching loss. On the other hand, since SBD is a unipolar type that operates only on the electrons, no recovery current is generated theoretically and the switching loss can be decreased drastically. However, when a high voltage SBB is intended to be attained by Si, since the dielectric breakdown field strength is small, the thickness of the drift layer increases and the concentration of impurity is lowered. As a result, since the device resistance increases remarkably, it is not suitable to practical use. On the other hand, since SiC has high dielectric breakdown strength, high performance SBD with low device resistance even at high withstanding voltage can be attained. However, SBD involves a problem that reverse leak current is large in view of the structure.
The leak current can be decreased by decreasing the electric field strength at the Schottky interface. For this purpose, a junction barrier Schottky (JBS) structure of hybridizing a pn junction to the Schottky interface of SiC has been proposed. Upon application of a reverse voltage, a depletion layer diffuses from a pn junction to exhibit pinch-off below the Schottky junction region. Therefore, an electric field at the Schottky junction interface is decreased and the leak current can be decreased.
Japanese Unexamined Patent Application Publication No H05 (1993)-136015 discloses a junction barrier Schottky diode. The structure is shown in FIG. 17. FIG. 17 shows an n+ SiC substrate 1 comprising SiC, an n− SiC drift layer 2, Schottky junction region 3, a p+ SiC region 4, and an anode electrode 5 and a cathode electrode 6 of the Schottky diode, respectively. Further, at the surface of the n− SiC drift layer, the width of the p+ SiC region is shown by Lpn and the distance between adjacent p+ SiC regions is shown by LSBD.
In the JBS structure, for decreasing the electric field strength at the Schottky junction interface further, it is necessary to pinch off the area below the Schottky junction region at a lower reverse voltage, that is, it is necessary to make the Schottky region smaller. However, as the Schottky region decreases, since the current path is narrowed, the on resistance increases. Therefore, the electric field strength at the Schottky junction interface and the on resistance are in a trade-off relation. For improving the trade-off, a trench type junction barrier Schottky diode having a plurality of trenches formed in the surface of the drift layer and p+ SiC region disposed at the inner wall thereof is effective.
Japanese Unexamined Patent Application Publication No H04(1992)-321274 discloses a trench type junction barrier Schottky diode. FIG. 18 shows a cross sectional structural view of a trench type junction barrier Schottky diode. A p+ SiC region 4 is formed to the inner wall of the trench 7. In FIG. 18, portions carrying same references as those in FIG. 17 show identical portions in which an opening width of the trench 7 is shown as Lpn and distance between each of the trenches is shown as LSBD. 